Trichlorfon, o,o-dimethyl 2,2,2-trichloro-l-hydroxyethyl-phosphonate, an organophosphate pesticide and vermifuge has been used to treat laboratory mice, rats, and hamsters infected with the pinworm, Syphacia obvelata. A concern was the effect of this drug on the animal's immune system. The immunotoxicity of this drug was examined in B6C3F1 mice. IgM antibody production against sheep red blood cells, delayed hypersensitivity to keyhole limpet hemocyanin, and the lymphoproliferative response to E. coli lipopolysaccharide were used to assay B cell function. T cell function was measured by responses to concanvalin A and phytohemagglutinin. No significant effects were observed using the above criteria. A statistically significant increase in liver and kidney but not spleen weight and organ to body weight ratio was observed. These weight differences are thought to be due to the decreased water intake in the drug treated group. The significance of this project lies in the fact that trichlorfon kills the parasite and is more efficacious than the more commonly used alternative, piperazine, which does not kill the parasite. This data suggests that the use of trichlorfon in mice will not compromise the host's immune system. The increased liver and kidney weights will be examined in a second group of mice.